With the development of semiconductor technology, the sizes of transistors continue to shrink, thereby causing the speed of devices and systems to increase. In a transistor with smaller sizes, the thickness of the gate dielectric layer such as a SiO2 layer becomes thinner. However, when the thickness of the SiO2 layer decreases to a certain degree, the SiO2 layer will not be able to provide a good insulation function, and it is easy to cause a leakage current from the gate to active regions. This substantially degrades the performance of the device.
A gate stack structure of high-K material/metal has been proposed to replace the conventional gate stack structure of SiO2/polysilicon. The high-K material is such a material, the dielectric constant K of which is larger than 3.9. For example, the high-K material may include HfO2, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, Al2O3, or La2O3, etc. The above-mentioned leakage current problem can be overcome by a great degree with the use of the high-K material as the gate dielectric layer.
It is known that the threshold voltage (Vt) of a transistor can be effectively reduced by adding materials such as La into the material of the gate dielectric layer. This facilitates improving the performance of the device. However, the effectiveness of the materials such as La in reducing the threshold voltage Vt is affected by many factors. For example, reference 1 (M. Inoue et al, “Impact of Area Scaling on Threshold Voltage Lowering in La-Containing High-k/Metal Gate NMOSFETs Fabricated on (100) and (110) Si”, 2009 Symposium on VLSI Technology Digest of Technical Papers, pp. 40-41) discussed the effect of La in detail and found that there is a strong narrow channel effect, in which the effect of La becomes lower as the width of the gate becomes narrower, and a strong corner effect, in which the effect of La is affected by the round corners of the channel region.
As the channel becomes narrower, the effectiveness of the gate dielectric layer is affected across the channel region, and thus it is necessary to adopt other measures to efficiently deal with the reduction of the threshold voltage Vt.